A number of anti-cancer drug are currently in the market for the treatment of various cancers. For example, paclitaxel and docetaxel are two promising anti-cancer drugs used to treat breast and ovarian cancers, and which hold promise for the treatment of various other cancers such as skin, lung, head and neck carcinomas. Other promising chemotherapeutic agents are being developed or tested for treatment of these and other cancers. Compounds such as paclitaxel, docetaxel and other taxanes, camptothecins, epothilones and quassinoids, as well as other compounds exhibiting efficacy in cancer treatment, are of considerable interest. Of special interest are natural product drugs and their synthetic analogs with demonstrated anticancer activity in vitro and in vivo.
However, many identified anti-cancer compounds present a number of difficulties with their use in chemotherapeutic regimens. A major problem with the use of such chemotherapeutic agents in cancer treatment is the difficulty targeting cancer tissues, without adversely affecting normal, healthy tissues. For example, paclitaxel exerts its antitumor activity by interrupting mitosis and the cell division process, which occurs more frequently in cancer cells, than in normal cells. Nonetheless, a patient undergoing chemotherapy treatment may experience various adverse effects associated with the interruption of mitosis in normal, healthy cells.
Targeted cancer therapies that can selectively kill cancer cells without harming other cells in the body would represent a major improvement in the clinical treatment of cancer. Reports of targeting drugs using antibodies have appeared in the literature since 1958. Targeting drugs by conjugation to antibodies for selective delivery to cancer cells has had limited success due to the large size of antibodies (MW=125-150 kilodaltons) and thus their relative inability to penetrate solid tumors.
An alternative strategy comprises the use of smaller targeting ligands and peptides, which recognize specific receptors unique to or overexpressed on tumor cells, as the targeting vector. Such constructs have molecular weights of 2-6 kilodaltons, which allow ready penetration throughout solid tumors.
Accordingly, it would be highly desirable to develop novel compounds and methods for use in directly targeting cancer cells with chemotherapeutic agents in cancer treatment regimens. This, in turn, could lead to reduction or elimination of toxic side effects, more efficient delivery of the drug to the targeted site, and reduction in dosage of the administered drug and a resulting decrease in toxicity to healthy cells and in the cost of the chemotherapeutic regimen.
One particular approach of interest is the use of anticancer drug moieties that have been conjugated to tumor molecules. For example, U.S. Pat. No. 6,191,290 to Safavy discusses the formation and use of a taxane moiety conjugated to a receptor ligand peptide capable of binding to tumor cell surface receptors. Safavy in particular indicates that such receptor ligand peptides might be a bombesin/gastrin-releasing peptide (BBN/GRP) receptor-recognizing peptide (BBN [7-13]), a somatostatin receptor-recognizing peptide, an epidermal growth factor receptor-recognizing peptide, a monoclonal antibody or a receptor-recognizing carbohydrate.
One important aspect of synthesizing these drug molecular conjugates is connecting these two units with a linker or linkers that provide conjugates with desired characteristics and biological activity, in particular, a conjugate that is stable in systemic circulation but releases cytotoxic agent once internalized into cancer cells or concentrated in the locally acidic tumor environment. Such an agent would be expected to exhibit lower toxicity to normal tissues. The resulting conjugate should also be sufficiently stable until it reaches the target tissue, and thus maximizing the targeting effect with reduced toxicity to normal, healthy tissue.
The blood-brain barrier (BBB) is a specialized physical and enzymatic barrier that segregates the brain from systemic circulation. The physical portion of the BBB is composed of endothelial cells arranged in a complex system of tight junctions which inhibit any significant paracellular transport. The BBB functions as a diffusion restraint selectively discriminating against substance transcytosis based on lipid solubility, molecular size and charge thus posing a problem for drug delivery to the brain. Drug delivery across the BBB is further problematic due to the presence of a high concentration of drug efflux transporters (e.g., P-glycoprotein, multi-drug resistant protein, breast cancer resistant protein). These transporters actively remove drug molecules from the endothelial cytoplasm before they even cross into the brain.
The methods that are currently employed for drug delivery in treatment of brain malignancies are generally nonspecific and inefficient. An additional problem to consider when treating brain diseases is the diffusion of the drug in its vehicle across the tumor or affected tissue. Mostly the size, as well as other physiologic characteristics of the vehicles that are currently in use for such delivery of drugs to the brain, hamper efficient diffusion of the drug through the diseased tissue. The lack of efficient drug diffusion affects the efficacy of the treatment.
Peptides have been extensively studied as carrier molecules for drug delivery to the brain in hope they could be employed as drug delivery vehicles. Peptides are, however, problematic due to their limited bioavailability. Even though methods to increase the bioavailability of such molecules have been intensively explored, they resulted in modest success at best.
Increased cell proliferation and growth is a trademark of cancer. The increase in cellular proliferation is associated with high turnover of cell cholesterol. Cells requiring cholesterol for membrane synthesis and growth may acquire cholesterol by receptor mediated endocytosis of plasma low density lipoproteins (LDL), the major transporter of cholesterol in the blood, or by de novo synthesis.
LDL is taken up into cells by a receptor known as the LDL receptor (LDLR); the LDL along with the receptor is endocytosed and transported into the cells in endosomes. The endosomes become acidified and this releases the LDL receptor from the LDL; the LDL receptor recycles to the surface where it can participate in additional uptake of LDL particles. There is a body of evidence that suggests that tumors in a variety of tissues have a high requirement for LDL to the extent that plasma LDLs are depleted. The increased import of LDL into cancerous cells is thought to be due to elevated LDL receptors (LDLR) in these tumors. Some tumors known to express high numbers of LDLRs include some forms of leukemia, lung tumors, colorectal tumors and ovarian cancer.
Comparative studies of normal and malignant brain tissues have shown a high propensity of LDLRs to be associated with malignant and/or rapidly growing brain cells and tissues. Some studies suggest that rapidly growing brain cells such as those seen in early development and in aggressively growing brain tumors exhibit increased expression of LDLRs due to their increased requirement for cholesterol.
Among the problematic and inefficiently treated brain cancers is glioblastoma multiforme (GBM). This devastating brain tumor is 100% fatal. Moreover, over 85% of total primary brain cancer-related deaths are due to GBM. Current therapies rely on a multimodal approach including neurosurgery, radiation therapy and chemotherapy. Even the best efforts using these approaches have resulted in only a modest increase in survival time for patients afflicted with this tumor.
GBM being gliomas of the highest malignancy is characterized by uncontrolled, aggressive cell proliferation and general resistance to conventional therapies. GBM cells in culture have high numbers of low density lipoprotein receptors (LDLR). Since this receptor is nearly absent in neuronal cells and normal glial cells, it represents an ideal target for the delivery of therapeutic agents such as cytotoxins or radiopharmaceuticals. Efforts to improve existing therapies or to develop new ones have not been successful and the outcome of treatment for malignant gliomas is only modest, at best, with a median survival time of approximately 10 months.
Unlike normal brain cells that have few LDL receptors, GBM cells in culture have high numbers of LDL receptors on their surface. Other cancers are likely to also have high expression of LDLR due to the highly proliferative nature of the cancerous tissue and need for cholesterol turnover. This suggests that the LDL receptor is a potential unique molecular target in GBM and other malignancies for the delivery of anti-tumor drugs via LDL particles.
Maranhäo and coworkers have demonstrated that a cholesterol-rich microemulsion or nanoparticle preparation termed LDE concentrates in cancer tissues after injection into the bloodstream. D. G. Rodrigues, D. A. Maria, D. C. Fernandes, C. J. Valduga, R. D. Couto, O. C. Ibanez and R. C. Maranhäo Improvement of paclitaxel therapeutic index by derivatization and association to a cholesterol-rich microemulsion: in vitro and in vivo studies. Cancer Chemotherapy and Pharmacology 55: 565-576 (2005). The cytotoxicity, pharmacokinetics, toxicity to animals and therapeutic action of a paclitaxel lipophilic derivative associated to LDE were compared with those of commercial paclitaxel. Results showed that LDE-paclitaxel oleate was stable. The cytostatic activity of the drug in the complex was diminished compared with the commercial paclitaxel due to the cytotoxicity of the vehicle Cremophor EL used in the commercial formulation. Maranhäo and coworkers showed LDE-paclitaxel oleate is a stable complex and compared with paclitaxel, toxicity is considerably reduced and activity is enhanced which may lead to improved therapeutic index in clinical use.
Capturing the great potential of selective and specific delivery of chemotherapeutic compounds to cancer tissues via their over expression of LDL receptors and consequent high uptake of LDL particles from the systemic circulation, requires that the cancer chemotherapeutic agent have high lipophilicity so as to remain entrapped in the lipid core of the LDL particle and not diffuse into the plasma to lead to toxic side effects from exposure of normal tissues to the agent. Further, once the LDL particle with its chemotherapeutic payload has entered the cancer cell via LDL receptor mediated uptake into the acidic environment of the endosome, the LDL receptor is disassociated from the LDL particle and is recycled to the cell surface and the LDL particle releases its lipid contents and its lipophilic chemotherapeutic agent to the enzymes and acidic environment of the endosome. Few cancer chemotherapeutic agents are intrinsically sufficiently lipophilic to be retained adequately within the lipid core of the LDL particle. This creates a need for suitable lipophilic derivatives of the cancer chemotherapeutic agent which have high stability in normal systemic circulation and retention in the lipid core of the LDL particles but readily release the active chemotherapeutic agent in the acidic environment of the endosome. The compounds of the present invention address this need.